1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly, to a technique of effecting a screening of a static random access memory (SRAM).
2. Description of the Related Art
With a recent demand for a high speed operation and large capacity of computer systems, the degree of integration of semiconductor memory devices constituting a computer system has been increasingly heightened and the operation speed thereof has also been increased. In such semiconductor memory devices, a screening test for rejecting or removing products having latent defects is carried out to prevent their initial failures. For example, with respect to SRAM devices as semiconductor memory devices constituting a computer system, the screening test is carried out using a burn-in apparatus, in which a power supply voltage higher than a normal voltage used in a read/write operation is applied across a pair of bit lines in each device.
On the other hand, a known typical SRAM device is constituted such that, when information is read out from a selected memory cell, a potential difference between a pair of bit lines corresponding to the memory cell is made small. This is intended to increase the read speed of information.
Accordingly, where such SRAM devices are subject to the above screening test, a problem occurs in that it is difficult or impossible to reject semiconductor memory devices having latent defects as "no good". This can occur, for example, in the case that electrically conductive foreign materials, the surface of which being oxidized, such as aluminium (Al) crumbs, silicon (Si) crumbs or the like resulting from the process of constructing the SRAM device remain adhering to a pair of complementary bit lines or a pair of bit lines across adjacent cells. Namely, since the potential difference between the pair of complementary bit lines is selected to be small to realize an increase in the information read speed, it is difficult to apply a sufficiently high voltage stress to the electrically conductive foreign materials even if a higher power supply voltage than the normal voltage is applied to the device in the screening test. As a result, it becomes impossible to reject the "no good" device with height reliability.